Drug and alcohol poisoning, suicide, and liver disease are depressing the life expectancy of white people in the United States, according to a new government study, explaining why the longevity gains experienced by other ethnic groups in recent years haven’t been realized universally.

Researchers at the Centers for Disease Control and Prevention identified those three causes — along with Alzheimer’s disease and hypertension — as the main culprits of the slow growth in white people’s life spans when compared to black and Hispanic people over the last 15 years. The study did not include data for other ethnic groups.

From 2000 to 2014, life expectancy for black people increased 3.6 years, and for Hispanic people it went up 2.6 years, according to the CDC’s National Center for Health Statistics. But for white people, their life expectancy increased only 1.4 years over that time. And in fact, between 2013 and 2014 the measure actually declined slightly for white people for the first time in over two decades.

The study, published Friday as a NCHS Data Brief, follows recent research that showed more white people are dying young from specific causes such as drug overdoses and suicide.

The question was: “What effect is that having on life expectancy?’’ said Kenneth Kochanek, one of the NCHS researchers.

Combined, unintentional poisonings, suicide, and chronic liver disease drove a staggering 28 percent increase in the death rate of white people aged 25 to 34, as well as more modest increases in mortality for whites aged 35 to 54 over the 15-year study period.

Unintentional poisonings — the majority of which are caused by drugs or alcohol — had the biggest effect, knocking off around four months from the life expectancy of white people.

That led to a plateau in life expectancy for non-Hispanic white people. In 2013, the expected life span for this group was 81.2 years. In 2014, the number went down to 81.1.

“We’ve been looking at this upward trend for 20 years, and now all of a sudden we’re flatlining,’’ Kochanek said. “It’s like: ‘Okay, something’s going on.’’’

The trend defies history.

“It’s supposed to keep going up,’’ Kochanek said.